This invention relates to color image processing. More particularly, this invention relates to methods and apparatus for profiling color output devices.
A profile for a color output device (referred to herein as a “color profile”), includes transformations between the color space of the color output device and a profile connection space, such as Commission Internationale de l'Eclairage (“CIE”) XYZ, or other similar profile connection space. A color profile typically is produced in accordance with the profile specification of the International Color Consortium (“ICC”), and hence is referred to as an “ICC profile.” A profile connection space derived from the XYZ color space is commonly known as the CIE LAB color space, which expresses color values in a rectangular coordinate system, with L, a, and b values each corresponding to one of the three dimensions in the system.
An ICC profile generally includes a transform from the profile connection space to the device space (the “forward transform”), and a transform from the device space to the profile connection space (the “backwards transform”). For example, if a color output device is a four-color printer that uses cyan, yellow, magenta and black (“CMYK”) colorants, the forward transform may be used to convert device-independent LAB colorant values to equivalent device-dependent CMYK colorant values, and the backwards transform may be used to convert device-dependent CMYK colorant values to device-independent LAB colorant values.
To create a color profile for a color output device, such as a printer, copier, offset press, or other similar color output device, a test chart including numerous color patches is printed on media, such as paper, by the color output device. Exemplary test charts include IT8, ECI2002, and other similar test charts. Next, a measurement device, such as a calorimeter, spectrophotometer, spectrocolorimeter, or other similar device is used to obtain spectral or colorimetric data that describes each printed patch. The measured data are then provided to profile generation software, which calculates a color profile based on the spectral or colorimetric measurements. Profile generation software may be any conventional profile generation software, such as the EFI Color Profiler software, licensed by Electronics for Imaging, Inc., Foster City, Calif., U.S.A., or other similar profile generation software.
Previously known techniques for creating color profiles have several disadvantages. First, conventional test charts include a large number of color patches, and therefore are time-consuming to measure. Indeed, a conventional IT8 test chart includes more than 900 test patches, and a conventional ECI2002 test chart includes more than 1,400 color patches. In addition, because such a large number of measurements must be taken, the risk of measurement errors becomes significantly increased.
Second, conventional test charts use a relatively large amount of space on the output media, and therefore may require a special print run solely for purposes of printing the test charts. Indeed, a conventional IT8 test chart may be more than 600 cm2 and an ECI2002 test chart may be more than 1,200 cm2. Because such test charts often are too large to print alongside a print job, it may be very costly to print test charts on the color output device. For example, to print a conventional test chart on an offset printing press, the press typically must be taken offline, special plates must be installed on the press that include images of the test chart, the press must be restarted, and the test chart must be printed on several thousand sheets of output media. Such a process not only consumes valuable resources such as output media and ink, but also constitutes costly downtime during which the press is not being used for revenue-generating operations.
In view of the foregoing, it would be desirable to provide methods and apparatus for reducing the number of color samples that must be printed when profiling a color output device.
It further would be desirable to provide methods and apparatus for reducing the time required to profile a color output device.
It also would be desirable to provide methods and apparatus for reducing the risk of measurement error while profiling a color output device.
It additionally would be desirable to provide methods and apparatus for reducing the size of a test chart that must be printed while profiling a color output device.